There used to be washers with different size holes available to replace the thermostat. I've googled for them, but can't seem to come up with them. Maybe someone else on the board is more proficient at it than I am.

If nobody answers, use your micrometer to measure the thermostat thickness at the outer diameter where it is sorta crimped. Obtain some copper sheet of that thickness and cut out a washer to fit in the recess. Drill a 5/8 inch hole in the middle.